In a frequency selective fading channel, different subcarriers will experience different channel gains. In traditional Orthogonal Frequency Division Multiplex (OFDM) time division multiple access (TDMA) systems each user transmits on all sub-carriers simultaneously and users share the channel in time. If the channel is perfectly known to the transmitter, the so-called “water-filling” policy (that is, more power when the channel gain is high and less power when the channel gain is low) is known to be optimal in maximising the data rate. The increase of data rate, by using transmit power allocation in a single user OFDM system is due to spectral diversity effects.
In Orthogonal Frequencv Division Multiplex Access (OFDMA) systems, users do not share the channel in time but in frequency by transmitting on a (typically mutually exclusive) sub-set of the available sub-carriers. A set of sub-carriers may be grouped into a sub-channel with sub-channels allocated to users. Although the water filling principle still applies, the sharing of sub-carriers introduces an additional degree of freedom to the allocation of system resources for multiple access. Accordingly, it is necessary to consider different solutions for the problem of subcarrier and power allocation in an OFDMA, or multi-user OFDM system. It is likely that signals from different users will undergo independent fading because the users are likely not to be in the same location. Therefore, the probability of all the users' signals on the same subcarrier being subject to significant fading is very low. In a multi-user OFDM system, the data rate can be increased by exploiting multi-user diversity.
In OFDMA, allocating one subcarrier to one user typically prevents other users from using that subcarrier, since it is desirable to avoid the interference that arises when users share the same subcarrier. Hence, the optimal solution is not necessarily to assign the best subcarriers seen by a single chosen user (as in a single user system). This may be the case because, for example, it may happen that the best subcarrier of one user is also the best subcarrier for another user who happens to have no other good subcarriers. Hence, a different approach should be considered.
As stated above, an OFDMA system provides an extra degree of freedom since there are multiple sub-carriers available to be allocated, and it is this property that can be exploited. OFDMA matches well to the multi-user scenario; a subcarrier which is of low quality to one user can be of high quality to another user and can be allocated accordingly. By adaptively assigning sub-carriers, it is possible to take advantage of channel diversity among users in different locations. This “multi-user diversity” stems from channel diversity including independent path loss and fading of users. Previously considered solutions suggested possible subcarrier and power allocation algorithms for OFDMA systems. For example, see W. Rhee, J. M. Cioffi, “Increase in capacity of multi-user OFDM system using dynamic subchannel allocation”, Vehicular Technology Conference Proceedings, 2000, IEEE 51st, Volume: 2, 15-18 May 2000, Page(s): 1085-1089; and J. Jang, K. Bok Lee, “Transmit Power Adaptation for Multi-user OFDM Systems”, IEEE Journal on Selected Areas in Communications, Volume: 21, Issue: 2, February 2003, Page(s): 171-178.
FIG. 1 illustrates channel diversity in a radio frequency telecommunications system. A base station 1 operates to transmit signals to mobile receivers 21 and 22. Obstacles, such as buildings 3, can cause the transmissions between the base station 1 and the mobile users 21 and 22 to take multiple paths 41 and 42. This phenomenon is well known and is known as multipath diversity. The signals arriving at the mobile receivers will vary in gain as a function of frequency, due to the varying lengths of the paths and reflections occurring on those paths. This means that different users receive different gain values for different subcarriers.
FIG. 2 illustrates a base station suitable for use in an OFDM or OFDMA system. The base station 10 receives data inputs U1, U2 . . . UK from a plurality of users at an encoder 102. The encoder 102 encodes these user data signals U1 to UK onto subcarriers C1, C2 . . . CN. A controller 108 allocates the subcarriers C1 to CN to the users U1 to UK. A transmitter transformation unit 104 takes the subcarrier signals and applies an inverse fast Fourier transform (IFFT) and a parallel-to-serial conversion to produce a serial output data stream. This data stream is supplied to an output unit 106, which adds a cyclic prefix and converts the digital signal to analogue for transmission from an antenna 20, in known manner.
The controller 108 receives feedback signals F1 to FK indicative of the channel subcarrier performance for each user. The controller 108 supplies control signals 110 on a control channel to the mobile receivers.
FIG. 3 illustrates a receiver for use in an OFDM system. Each user has a receiving antenna 30 connected to a user receiver 40. The user receiver 40 includes an input unit 402 which performs analogue-to-digital conversion of the incoming signal and removes the cyclic prefix which was added by the base station transmitter. The digital signal is then processed by a receiver transformation unit which applies a fast Fourier transform (FFT) and serial-to-parallel conversion to produce subcarrier signals C1 to CN. The subcarrier signals are received by a subcarrier selector 406 which, in dependence on received controlled signals 410, selects the subcarriers for the user K. These subcarriers are supplied to a decoder 408 which decodes the data signal relating to user K, to produce an output signal DK.
Operation of the base station of FIG. 2 and the receiver of FIG. 3 will now be explained.
The controller 108 at the base station 10, having determined channel information from all users, allocates subcarriers to each user according to a subcarrier allocation algorithm. When the CSI (Channel State Information) is available at the transmitter, the transmitter can assign subcarriers to users and also adapt the transmit power in a symbol by symbol (or packet by packet) manner to increase data rate, assuming that the fading characteristics of the channel are constant for the symbol (or packet) duration.
In one previous solution mentioned, Rhee and Cioffi showed that since each subcarrier is assigned to a user whose channel gain is good for that subcarrier, there is no need for different power allocation among the subcarriers. Hence, the algorithm only needs to find the most appropriate subcarriers for all users and allocate equal power; this results in lower implementation complexity.
In the other previous solution mentioned, Jang and Bok Lee propose a transmit power allocation scheme and subcarrier allocation algorithm in the general case where users are allowed to share a subcarrier. In that case, if the transmit power for a specific users' signal is increased, the interference to other users' signals on the same subcarrier is also increased. However, after a mathematical analysis, it was found that the capacity is maximised if a subcarrier is assigned to only one user and hence no interference occurs (something that has become a fundamental assumption for all previous and future work). As with Rhee and Cioffi, it was found that equal power allocation is the best approach since water-filling over the allocated subcarriers will not give any significant gain and will increase computational complexity. In the final proposed scheme, only one user who has the best channel gain for that subcarrier transmits data at that subcarrier (for each subcarrier check which user has the best gain). It was also found that data rate is increased for increased number of users since it provides more multi-user diversity.
Although in the scheme proposed by Jang and Bok Lee the received average SNR for each subcarrier is increased and the average data rate is increased, this algorithm is not fair for the users. The number of subcarriers assigned to each user is not fixed, hence each user can have different data rates. Additionally, if it happens that one user does not have the best channel gain of the multiple users being considered for any of the subcarriers (for example, due to its location) then no subcarriers at all will be allocated to that user.